Testing of microwave devices often involves passing a high-power microwave signal through a device or system under test (“DUT”), and recording the response of the DUT to changes in the signal or to changes in the configuration of the DUT. For instance, a frequency response of the DUT may be obtained by passing a microwave signal through a device and sweeping the signal throughout a frequency range. Or, linearity of an amplifier may be measured by passing a microwave signal at a known frequency through a DUT, and adjusting the input power of the microwave signal through a known power range while monitoring the output power.
When testing, it is desirable to minimize artifacts caused by test fixtures, and to calibrate the test fixture so that any remaining data artifacts caused by test fixtures can be minimized or eliminated in the measured data. One technique is to connect the output of the DUT to a microwave termination, typically having an impedance matched to 50 ohms. Such an impedance of the termination will reduce reflections caused by an impedance mismatch between the DUT and the termination. Energy absorbed by the microwave termination is dissipated as heat.
In other situations, when stopping the delivery of microwave power temporarily, it may be desirable to shunt a microwave signal to a load rather than to turn off temporarily the microwave source. For instance, some microwave sources such as a traveling wave tube amplifier (“TWTA”) are known to require a warm-up time to achieve full rated performance. Therefore, turning off such a microwave source may affect how quickly the source can be turned back on. Furthermore, a temporary open circuit may cause power reflections that damage the microwave source. Instead, when stopping the delivery of microwave power, the power can be shunted to a microwave termination while the microwave source remains active.
Testing at high microwave powers presents certain engineering challenges. Nonlinear effects generally become relatively more pronounced as the input power increases. For instance, it is well known that passive intermodulation products may be formed in a passive microwave device, in which an input microwave signal having two or more frequency components may generate spurious output signals having frequencies that are at sums and differences of multiples of the frequencies of the input signals. As the power of the input signal increases, the power of the spurious signals increases at a faster rate. Therefore, it is desirable to provide a microwave termination suitable for use in high power microwave usage.